Conservation of Energy
Posted July 23, 2008on:
This is an experiment I did in my Conceptual Physics class to demonstrate Conservation of Energy. I take a clean plastic peanut butter jar and put in about an inch of copper BB’s. We then wrap the jar in a towel and have a student start shaking the jar as hard as they can. The towel serves two purposes; it stops the transfer of heat from the students’ hands to the jar, and it helps deaden the deafening sound this little activity generates. Use the non-contact thermometer to take a reading of the BB’s before beginning and then after every two minutes of shaking. I usually set up two or three jars and have the students pass the jar to someone else after the temperature is taken. Chart the temperature on the board. Don’t expect to get anything else done while this is happening. There is too much noise and everybody wants to see how vigorously they can shake the jars.
The students are putting energy into the system because they are shaking the jar. They are actually taking the chemical energy from the food they ate and turning that into kinetic energy. The kinetic energy is transfered into themal energy (heat) because of the friction of the BB’s banging into each other. I just thought of this, but it would be a good idea to have another jar set up as a control.